PERLBOOT
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perlboot - Beginner's Object-Oriented Tutorial
If you're not familiar with objects from other languages, some of the other Perl object documentation may be a little daunting, such as perlobj, a basic reference in using objects, and perltoot, which introduces readers to the peculiarities of Perl's object system in a tutorial way.
So, let's take a different approach, presuming no prior object experience. It helps if you know about subroutines (perlsub), references (perlref et. seq.), and packages (perlmod), so become familiar with those first if you haven't already.
If we could talk to the animals...
Cow::speak;
Horse::speak; Sheep::speak;
a Horse goes neigh! a Sheep goes baaaah!
@pasture = qw(Cow Cow Horse Sheep Sheep); foreach $animal (@pasture) {
a Cow goes moooo! a Horse goes neigh! a Sheep goes baaaah! a Sheep goes baaaah! Wow. That symbolic coderef de-referencing there is pretty nasty. We're counting on no strict subs mode, certainly not recommended for larger programs. And why was that necessary? Because the name of the package seems to be inseparable from the name of the subroutine we want to invoke within that package.
Or is it?
Introducing the method invocation arrow
For now, let's say that Class- invokes subroutine method in package Class. (Here, ``Class is used in its ``category meaning, not its ``scholastic'' meaning.) That's not completely accurate, but we'll do this one step at a time. Now let's use it like
Cow-
a Horse goes neigh! a Sheep goes baaaah!
Ahh! Now that the package name has been parted from the subroutine name, we can use a variable package name. And this time, we've got something that works even when use strict refs is enabled.
Invoking a barnyard
Let's take that new arrow invocation and put it back in the
@pasture = qw(Cow Cow Horse Sheep Sheep);
foreach $animal (@pasture) { $animal- There! Now we have the animals all talking, and safely at that, without the use of symbolic coderefs.
But look at all that common code. Each of the speak routines has a similar structure: a print operator and a string that contains common text, except for two of the words. It'd be nice if we could factor out the commonality, in case we decide later to change it all to says instead of goes.
And we actually have a way of doing that without much fuss, but we have to hear a bit more about what the method invocation arrow is actually doing for us.
The extra parameter of method invocation
my $class = shift; print
my $class = shift; print In each case, $class will get the value appropriate for that subroutine. But once again, we have a lot of similar structure. Can we factor that out even further? Yes, by calling another method in the same class.
Calling a second method to simplify things
Let's call out from speak to a helper method called sound. This method provides the constant text for the sound itself.
{ package Cow;
sub sound { Now, when we call Cow-, we get a $class of Cow in speak. This in turn selects the Cow- method, which returns moooo. But how different would this be for the Horse?
{ package Horse;
sub sound { Only the name of the package and the specific sound change. So can we somehow share the definition for speak between the Cow and the Horse? Yes, with inheritance!
Inheriting the windpipes
We'll define a common subroutine package called Animal, with the definition for
sub speak { my $class = shift; print
@ISA = qw(Animal); sub sound { Note the added @ISA array. We'll get to that in a minute.
But what happens when we invoke Cow- now?
First, Perl constructs the argument list. In this case, it's just Cow. Then Perl looks for Cow::speak. But that's not there, so Perl checks for the inheritance array @Cow::ISA. It's there, and contains the single name Animal.
Perl next checks for speak inside Animal instead, as in Animal::speak. And that's found, so Perl invokes that subroutine with the already frozen argument list.
Inside the Animal::speak subroutine, $class becomes Cow (the first argument). So when we get to the step of invoking $class-, it'll be looking for Cow-, which gets it on the first try without looking at @ISA. Success!
A few notes about @ISA
This magical @ISA variable (pronounced ``is a not ``ice-uh), has declared that Cow ``is a'' Animal. Note that it's an array, not a simple single value, because on rare occasions, it makes sense to have more than one parent class searched for the missing methods.
If Animal also had an @ISA, then we'd check there too. The search is recursive, depth-first, left-to-right in each @ISA. Typically, each @ISA has only one element (multiple elements means multiple inheritance and multiple headaches), so we get a nice tree of inheritance.
When we turn on use strict, we'll get complaints on @ISA, since it's not a variable containing an explicit package name, nor is it a lexical (``my'') variable. We can't make it a lexical variable though (it has to belong to the package to be found by the inheritance mechanism), so there's a couple of straightforward ways to handle that.
The easiest is to just spell the package name
use vars qw(@ISA); @ISA = qw(Animal);
use Animal; use vars qw(@ISA); @ISA = qw(Animal);
use base qw(Animal); And that's pretty darn compact.
Overriding the methods
{ package Mouse; @ISA = qw(Animal); sub sound {
Mouse-
[but you can barely hear it!? Here, Mouse has its own speaking routine, so Mouse- doesn't immediately invoke Animal-. This is known as ``overriding''. In fact, we didn't even need to say that a Mouse was an Animal at all, since all of the methods needed for speak are completely defined with Mouse.
But we've now duplicated some of the code from Animal-, and this can once again be a maintenance headache. So, can we avoid that? Can we say somehow that a Mouse does everything any other Animal does, but add in the extra comment? Sure!
First, we can invoke the Animal::speak method
{ package Mouse; @ISA = qw(Animal); sub sound { Note that we have to include the $class parameter (almost surely the value of ) as the first parameter to Animal::speak, since we've stopped using the method arrow. Why did we stop? Well, if we invoke Animal- there, the first parameter to the method will be not , and when time comes for it to call for the sound, it won't have the right class to come back to this package.
Invoking Animal::speak directly is a mess, however. What if Animal::speak didn't exist before, and was being inherited from a class mentioned in @Animal::ISA? Because we are no longer using the method arrow, we get one and only one chance to hit the right subroutine.
Also note that the Animal classname is now hardwired into the subroutine selection. This is a mess if someone maintains the code, changing @ISA for Animal there in speak. So, this is probably not the right way to go.
Starting the search from a different place
A better solution is to tell Perl to search from a higher
{ package Mouse;
Ahh. This works. Using this syntax, we start with Animal to find speak, and use all of Animal's inheritance chain if not found immediately. And yet the first parameter will be $class, so the found speak method will get Mouse as its first entry, and eventually work its way back to Mouse::sound for the details.
But this isn't the best solution. We still have to keep the @ISA and the initial search package coordinated. Worse, if Mouse had multiple entries in @ISA, we wouldn't necessarily know which one had actually defined speak. So, is there an even better way?
The SUPER way of doing things
By changing the Animal class to the SUPER class in that invocation, we get a search of all of our super classes (classes listed in @ISA)
{ package Mouse;
So, SUPER::speak means look in the current package's @ISA for speak, invoking the first one found.
Where we're at so far...
and attempts to invoke
Class::method(
However, if Class::method is not found, then @Class::ISA is examined (recursively) to locate a package that does indeed contain method, and that subroutine is invoked instead.
Using this simple syntax, we have class methods, (multiple) inheritance, overriding, and extending. Using just what we've seen so far, we've been able to factor out common code, and provide a nice way to reuse implementations with variations. This is at the core of what objects provide, but objects also provide instance data, which we haven't even begun to cover.
A horse is a horse, of course of course -- or is it?
Let's start with the code for the Animal class and
sub speak { my $class = shift; print
But all of our Horse objects would have to be absolutely identical. If I add a subroutine, all horses automatically share it. That's great for making horses the same, but how do we capture the distinctions about an individual horse? For example, suppose I want to give my first horse a name. There's got to be a way to keep its name separate from the other horses.
We can do that by drawing a new distinction, called an ``instance. An ``instance is generally created by a class. In Perl, any reference can be an instance, so let's start with the simplest reference that can hold a horse's name: a scalar reference.
my $name =
This operator stores information about the package named Horse into the thing pointed at by the reference. At this point, we say $talking is an instance of Horse. That is, it's a specific horse. The reference is otherwise unchanged, and can still be used with traditional dereferencing operators.
Invoking an instance method
The method arrow can be used on instances, as well as names of packages (classes). So, let's get the sound that
To invoke sound, Perl first notes that $talking is a blessed reference (and thus an instance). It then constructs an argument list, in this case from just ($talking). (Later we'll see that arguments will take their place following the instance variable, just like with classes.)
Now for the fun part: Perl takes the class in which the instance was blessed, in this case Horse, and uses that to locate the subroutine to invoke the method. In this case, Horse::sound is found directly (without using inheritance), yielding the final subroutine
Note that the first parameter here is still the instance, not the name of the class as before. We'll get neigh as the return value, and that'll end up as the $noise variable above.
If Horse::sound had not been found, we'd be wandering up the @Horse::ISA list to try to find the method in one of the superclasses, just as for a class method. The only difference between a class method and an instance method is whether the first parameter is an instance (a blessed reference) or a class name (a string).
Accessing the instance data
Because we get the instance as the first parameter, we can now access the instance-specific data. In this case, let's
@ISA = qw(Animal); sub sound {
How to build a horse
Of course, if we constructed all of our horses by hand, we'd most likely make mistakes from time to time. We're also violating one of the properties of object-oriented programming, in that the ``inside guts'' of a Horse are visible. That's good if you're a veterinarian, but not if you just like to own horses. So, let's let the Horse class
@ISA = qw(Animal); sub sound {
Notice we're back to a class method, so the two arguments to Horse::named are Horse and Mr. Ed. The bless operator not only blesses $name, it also returns the reference to $name, so that's fine as a return value. And that's how to build a horse.
We've called the constructor named here, so that it quickly denotes the constructor's argument as the name for this particular Horse. You can use different constructors with different names for different ways of ``giving birth to the object (like maybe recording its pedigree or date of birth). However, you'll find that most people coming to Perl from more limited languages use a single constructor named new, with various ways of interpreting the arguments to new. Either style is fine, as long as you document your particular way of giving birth to an object. (And you were'' going to do that, right?)
Inheriting the constructor
But was there anything specific to Horse in that method? No. Therefore, it's also the same recipe for building anything else that inherited from Animal,
sub speak { my $class = shift; print Ahh, but what happens if we invoke speak on an instance?
my $talking = Horse-
Why? Because the Animal::speak routine is expecting a classname as its first parameter, not an instance. When the instance is passed in, we'll end up using a blessed scalar reference as a string, and that shows up as we saw it just now.
Making a method work with either classes or instances
All we need is for a method to detect if it is being called on a class or called on an instance. The most straightforward way is with the ref operator. This returns a string (the classname) when used on a blessed reference, and undef when used on a string (like a classname). Let's modify the name method first to
my $either = shift; ref $either ? $$either # it's an instance, return name :
my $either = shift; print $either- And since sound already worked with either a class or an instance, we're done!
Adding parameters to a method
sub named { my $class = shift; my $name = shift; bless $name, $class; } sub name { my $either = shift; ref $either ? $$either # it's an instance, return name :
an unnamed Sheep eats grass.
More interesting instances
What if an instance needs more data? Most interesting instances are made of many items, each of which can in turn be a reference or even another object. The easiest way to store these is often in a hash. The keys of the hash serve as the names of parts of the object (often called ``instance variables or ``member variables), and the corresponding values are, well, the values.
But how do we turn the horse into a hash? Recall that an object was any blessed reference. We can just as easily make it a blessed hash reference as a blessed scalar reference, as long as everything that looks at the reference is changed accordingly.
sub name { my $either = shift; ref $either ? $either-
sub named { my $class = shift; my $name = shift; my $self = { Name =
sub default_color {
sub default_color { Now, because name and named were the only methods that referenced the ``structure'' of the object, the rest of the methods can remain the same, so speak still works as before.
A horse of a different color
But having all our horses be brown would be boring. So let's add a method or two to get and set the color.
sub color { $_[0?-
Summary
So, now we have class methods, constructors, instance methods, instance data, and even accessors. But that's still just the beginning of what Perl has to offer. We haven't even begun to talk about accessors that double as getters and setters, destructors, indirect object notation, subclasses that add instance data, per-class data, overloading, ``isa and ``can tests, UNIVERSAL class, and so on. That's for the rest of the Perl documentation to cover. Hopefully, this gets you started, though.
For more information, see perlobj (for all the gritty details about Perl objects, now that you've seen the basics), perltoot (the tutorial for those who already know objects), perltootc (dealing with class data), perlbot (for some more tricks), and books such as Damian Conway's excellent Object Oriented Perl.
Some modules which might prove interesting are Class::Accessor, Class::Class, Class::Contract, Class::Data::Inheritable, Class::!MethodMaker? and Tie::!SecureHash?
Copyright (c) 1999, 2000 by Randal L. Schwartz and Stonehenge Consulting Services, Inc. Permission is hereby granted to distribute this document intact with the Perl distribution, and in accordance with the licenses of the Perl distribution; derived documents must include this copyright notice intact.
Portions of this text have been derived from Perl Training materials originally appearing in the Packages, References, Objects, and Modules course taught by instructors for Stonehenge Consulting Services, Inc. and used with permission.
Portions of this text have been derived from materials originally appearing in Linux Magazine and used with permission.
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